The present invention relates to a novel aromatic alkylation process using the a layered catalyst composition. The layered composition comprises an inner core such as cordierite, and an outer layer comprising a bound zeolite that is bonded to the inner core. The outer layer is bonded to the inner core using an organic bonding agent such as polyvinyl alcohol, so that the resulting layered composition is sufficiently attrition resistant for use in commercial aromatic alkylation processes.
The alkylation of aromatic hydrocarbons such as benzene using solid catalysts is a well-developed art that is practiced commercially in large scale industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethyl benzene, which is subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene), which is subsequently used in the production of phenol and acetone. Those skilled in the art are familiar with the general design and operation of such alkylation processes. For example, U.S. Pat. No. 4,051,191 depicts a typical flow scheme suitable for the production of cumene. Of course, both the ethyl benzene and cumene production processes have undergone continual improvement since their commercial introduction.
The performance of aromatic alkylation processes is influenced to a significant extent by the activity and selectivity of the catalyst in the operating environment of the process. Currently available catalysts for aromatic alkylation include those having considerable acidity such as aluminum chloride and zeolites. The characterization of solid materials in terms their acidic properties is described in detail in Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, pp. 151-153. Compared to aluminum chloride, zeolitic catalyst have certain advantages in alkylation processes, such as fewer problems related to corrosion and spent catalyst disposal.
Despite these advantages associated with the use of zeolites, however, there is also a considerable expense related to loading large quantities of zeolite- (e.g. beta zeolite-) containing catalysts into commercial reactor volumes. Furthermore, due to the nature of aromatic alkylation catalyst deactivation, the initial catalyst loadings in commercial fixed-bed operations generally contain far more zeolite than that which is needed at any given time to catalyze the alkylation reaction. In other words, only a small portion of the active zeolite sites in fresh catalyst loadings is utilized for the desired alkylation reaction, while catalyst beyond the easily recognizable alkylation exotherm is essentially not used for alkylation initially. Moreover, the active acid sites in this downstream portion of the catalyst bed tend to promote undesirable side reactions, such as the formation of oligomers and diphenyl alkanes.
Since an excess of reactive alkylation sites (i.e. the acid sites of the zeolite) are present during most of the catalyst lifetime, the reaction is said to be diffusion limited. The reaction rate is determined not by the absolute number of active sites, but by the rate of diffusion of aromatic compounds (e.g. benzene) and alkylating agents (e.g. ethylene) to these sites. The excess reactive sites become utilized only gradually as the catalyst deactivates and as the alkylation zone and consequently the catalyst bed exotherm migrate toward the reactor outlet.
To better utilize the active sites of the zeolite and thus conserve the total amount of this expensive catalyst component required for a commercial loading, the prior art has recognized that coating an inert core with a shell or layer of zeolitic material is desirable. For example, U.S. Pat. No. 4,283,583 teaches that a coated catalyst improves aromatic alkylation process economics, since only a fraction of the zeolite otherwise needed is loaded. Further advantages with this mode of operation include the reduction of byproduct make and increase in catalyst activity per gram of active component (i.e. zeolite).
The teachings of the ""583 patent and other prior art (e.g. U.S. Pat. Nos. 4,077,912 and 4,255,253), however, fail to explain a way to coat the base support material that results in a layered catalyst composition able to withstand prolonged exposure to commercial aromatic alkylation conditions without significant attrition. For example, the coated catalyst preparation method described in the ""583 patent involves wetting a base support material and rolling this support in zeolite powder. Furthermore, the use of atmospheric pressure and consequently essentially gas-phase processing conditions are taught in the ""583 patent and other references. In contrast, commercial alkylation technologies, including the above-mentioned ethylbenzene and cumene production processes are performed most economically at pressures sufficient to cause the reactants to be substantially liquid phase. Under these conditions, a strong bond between the outer, zeolitic layer and core material is critical to prevent elution or erosion of the outer layer into process streams. The fact that benzene and other aromatics used in the liquid feed to such processes are notably strong solvents further emphasizes the importance, in using a layered catalyst, of having a strong, attrition resistant bond between the outer layer and core.
U.S. Pat. No. 5,516,740 discloses coating an inert core with a slurry of catalytically active ingredients and a boehmite/pseudo boehmite solvent. After the coating step, the resulting particles are calcined to convert the boehmite to gamma alumina and provide a bond between the inner inert core and outer active components. The present invention, however, does not rely solely on a boehmite slurry to provide 1) the binding of the outer layer of catalytically active ingredients and 2) the bond between the outer layer and the inner core. Rather, applicants have found superior results related to both of these functions through the use of an additional organic bonding agent. The ""740 patent further discloses that the catalyst is used in an isomerization process.
U.S. Pat. No. 6,013,851 and U.S. Pat. No. 4,482,774 are directed to inner core materials that are limited to zeolites and crystalline materials, respectively. In the ""851 patent, the catalyst is prepared by heating a synthesis mixture having crystals of the core zeolite dispersed therein. Likewise, the catalyst of the ""774 patent is made by heating a solution having preformed particles of the core material (in this case crystalline silica, e.g. silicalite) contained therein. Here, the solution contains a silica source for a modified silica, an organic template, a source of a modifying element, and an alkali. The catalyst used in the hydrocarbon conversion process of the ""874 patent comprises a crystalline modified-silica zeolite overlying a silica core having substantially the same crystalline structure as the modified-silica zeolite. Among other differences, the catalyst used in the aromatic alkylation process of the present invention has an inner core that is not necessarily a crystalline material.
Finally, published application WO 98/14274 A1 provides a preparation technique for a shell material that is deposited onto and bonded with a (preferably inert) core material. The thin shell is formed by a deposition process that involves repeatedly applying and drying small quantities of a slurry containing the shell material. The slurry can be in the form of a colloidal dispersion, or sol. The catalyst particles formed may be used for a wide variety of chemical reactions including alkylation.
In contrast to the prior art, applicants have developed a layered catalyst composition for the alkylation of aromatic hydrocarbons that differs from the prior art in several respects. The composition comprises an inner core such as cordierite and an outer layer comprising a bound zeolite. The outer layer is bonded to the inner core such that the attrition loss is less than about 25% of the weight of the outer layer. This attrition resistant catalyst is prepared using an organic bonding agent such as polyvinyl alcohol that increases the bond between the layer and the inner core, allowing sustained use of the layered composition under commercial alkylation conditions.
As stated above, the catalyst of the present invention, based on its improved strength and bonding characteristics, is suitable for alkylation processes, and especially those where the alkylation reaction takes place substantially in the liquid phase. The advantages associated with using an improved, attrition resistant, layered catalyst formulation in an aromatics alkylation process, however, are not limited to the aforementioned reduced expenditure on zeolite, higher selectivity to alkyl aromatics, and more efficient catalyst utilization.
It is also an important consideration that a layered catalyst composition can assume a variety of possible shapes, (e.g. rings) because, in many cases, the inner core material (e.g. cordierite) is commercially available in several physical forms. Layered compositions, therefore, provide vastly greater possibilities than the uniform zeolite catalysts in terms of their physical form. This is evident because the zeolite itself, either alone or in combination with a binder, cannot be fabricated into the types of thin-walled structures obtainable as a deposited layer on an inner core, without compromising strength properties to the extent that the catalyst would be unfit for the alkylation reaction environment. In particular, the ability to use catalyst shapes having a sufficiently high bed voidage (i.e. greater than about 0.35) associated with rings and other shapes favorably impacts the pressure drop across the alkylation zone. Hence, the pumping energy requirement associated with recycling a portion of the alkylation reactor effluent back to the reactor inlet is reduced, resulting in further economic advantages. The use of high recycle operation (i.e. greater than about 3:1 by weight of reactor effluent to fresh feed) is beneficial for several reasons.
To illustrate the importance of recycling alkylation zone effluent, the prior art has recognized that minimizing the concentration of the alkylating agent (e.g. propylene) in the alkylation reaction zone further reduces the formation of undesirable byproducts. In this regard, U.S. Pat. No. 4,008,290 teaches that, as a result of suppressing the reactor inlet propylene concentration in a cumene production process, the formation of propylene oligomers is limited and the reaction exotherm is reduced. The latter effect additionally lowers the productivity of byproducts that normally result from a high temperature rise across the alkylation zone or zones. Likewise, U.S. Pat. No. 5,877,370 recognizes that the byproduct 1,1 diphenylethane (1,1-DPE) concentration is decreased in an ethylbenzene production process when the reaction zone ethylene concentration is lowered.
In one embodiment, the present invention is a process for the alkylation of an aromatic hydrocarbon feed stream to yield an alkylated aromatic product, the process comprising contacting the feed stream with an alkylating agent at alkylation conditions in the presence of a layered composition comprising an inner core, an outer layer bonded to the inner core such that the attrition loss is less than about 25% of the weight of the outer layer, and the outer layer comprising a zeolite and a binder.
In a second embodiment, the present invention is a process for the alkylation of an aromatic hydrocarbon feed stream, the process comprising forming a combined stream comprising the feed stream, a recycle portion of an alkylation zone effluent stream, and an alkylating agent; alkylating the combined stream in an alkylation zone at alkylation conditions, the alkylation zone containing a layered composition catalyst comprising an inner core, an outer layer bonded to the inner core such that the attrition loss is less than about 25% of the weight of the outer layer, and the outer layer comprising a zeolite and a binder; recovering from the alkylation zone the alkylation zone effluent; separating a product portion of the alkylation zone effluent into a low-boiling fraction comprising benzene, a product stream comprising an alkylated aromatic product, and a high boiling fraction comprising polyalkylated aromatic compounds; and transalkylating at least a portion of the low-boiling fraction and at least a portion of the high boiling fraction in a transalkyation zone containing a transalkylation catalyst at transalkylation conditions.
In another embodiment, the present invention is a process as described above where the combined stream is substantially a liquid at alkylation conditions.
In yet another embodiment the present invention is a process for preparing a layered catalyst composition comprising an inner core, an outer layer bonded to the inner core, the outer layer bonded to the inner core such that the attrition loss is less than 25% of the weight of the outer layer, and the outer layer comprising a zeolite and a binder, the process comprising coating the inner core with a slurry comprising the zeolite, a sol of the binder, and an organic bonding agent, to yield a coated core having the outer layer, drying the coated core at a temperature from about 20xc2x0 C. to about 250xc2x0 C. to yield a dried coated core, and calcining the dried coated core at a temperature from about 400xc2x0 C. to about 900xc2x0 C. for a time sufficient to bond the outer layer to the inner core and provide the layered catalyst composition.
In a final embodiment the present invention is the product of the process described above.